<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>超时与网络异常</title>
</head>
<body>
  <button>点击发送请求</button>
  <div id="result"></div>
</body>
<style>
  #result {
    width: 200px;
    height: 100px;
    border: solid 1px #258;
  }
</style>
<script>
  const btn = document.getElementsByTagName('button')[0]
  const result = document.getElementById('result')
  btn.addEventListener('click', function(){
    const xhr = new XMLHttpRequest()
    // 超时设置2s
    xhr.timeout = 2000
    // 超时回调
    xhr.ontimeout = function () {
      alert('网络异常，请稍后重试！')
    }
    // 网络异常回调
    xhr.onerror = function () {
      alert('你的网络似乎出了点问题')
    }
    xhr.open('GET', 'http://127.0.0.1:8000/delay')
    xhr.send()
    xhr.onreadystatechange = function () {
      if(xhr.readyState === 4){
        if(xhr.status >= 200 && xhr.status < 300){
          result.innerHTML = xhr.response
        }
      }
    }
  })
</script>
</html>